Isatin: Identity with the Purified Endogenous Monoamine Oxidase Inhibitor Tribulin

Abstract: Purified tribulin, an endogenous monoamine oxidase (MAO) inhibitor, has been identified by direct probe insertion mass spectrometry as the indole‐2,3‐dione, isatin. A gas chromatographic‐mass spectrometric assay for isatin has been developed and used to measure its relatively high concentrations in unpurified human urine, and in rat heart and brain. Isatin is a known compound with a broad range of biological activity; this is the first report of its presence in the animal body. Isatin is a potent inhibitor of MAO, particularly of MAO B (IC 50 , 3 μ M ), and also binds to central benzodiazepine receptors (IC 50 against clonazepam, 123 μ M ).